Abstract

The Upper Miocene Senhata Formation is the infill of the Middle Miocene-Middle Pliocene forearc basin that developed in response to the north ward to northwestward subduction of the Philippine Sea plate beneath the Eurasia plate at the Sagami Trough. The formation has been affected by anticlinal and synclinal deformation and provides an opportunity for analyzing three-dimensional variations in thickness, stacking patterns, and bounding-surface features of component lithofacies associations. The lower part of the formation consists mainly of shoreface and shelf deposits and indicate falling-stage deposits that are overlain by transgressive shoreface and shelf deposits of the upper Senhata Formation, with the contact defined as a wave-ravinement surface. Forced-regressive shoreface-to-shelf deposits of the lower Senhata Formation are characterized by an erosional basal surface of a Type 1 sequence boundary and lateral change to multiply stacked, thicker regressive deposits with a gradational basal contact with the underlying shelf and upper slope deposits, indicating progradational and aggradational stacking of shoreface and shelf deposits, which are interpreted to be equivalent to the shelf-margin systems tract of a Type 2 sequence. The erosional sequence boundary at the base of forced-regressive deposits coincides laterally with a transgressive surface of erosion to the northwestern upslope area, where the forced-regressive deposits are absent. The lateral variation in the falling-stage regressive deposits of the lower Senhata Formation is interpreted to indicate that falling-stage and lowstand-stage deposits of Types 1 and 2 sequences are largely contemporaneous and their development is controlled by spatial variation in the rate of basin subsidence in both depositional-dip and depositional-strike directions.